Wireless communication systems have been widely used to provide various kinds of communication services such as voice or data services. Generally, a wireless communication system is a multiple access system that can communicate with multiple users by sharing available system resources (bandwidth, transmission (Tx) power, and the like). A variety of multiple access systems can be used. For example, a Code Division Multiple Access (CDMA) system, a Frequency Division Multiple Access (FDMA) system, a Time Division Multiple Access (TDMA) system, an Orthogonal Frequency Division Multiple Access (OFDMA) system, a Single Carrier Frequency-Division Multiple Access (SC-FDMA) system, a Multi-Carrier Frequency Division Multiple Access (MC-FDMA) system, and the like.
Conventionally, one transmission (Tx) antenna and one reception (Rx) antenna are used. MIMO technology is an abbreviation for Multiple Input Multiple Output technology. MIMO technology uses a plurality of transmission (Tx) antennas and a plurality of reception (Rx) antennas to improve the efficiency of transmission and reception (Tx/Rx) of data. In other words, MIMO technology allows a transmission end or reception end of a wireless communication system to use multiple antennas (hereinafter referred to as multi-antenna technology), so that capacity or performance can be improved. For convenience of description, the term “MIMO” can also be considered to be multi-antenna technology.
In more detail, MIMO technology is not dependent on a single antenna path to receive a single message. Instead, MIMO technology collects a plurality of data fragments received via several antennas, merges the collected data fragments, and completes total data. As a result, MIMO technology can increase a data transfer rate within a predetermined-sized cell region, or can increase system coverage while guaranteeing a specific data transfer rate. Under this situation, MIMO technology can be widely applied to mobile communication terminals, repeaters, or the like. MIMO technology can extend the range of data communication, so that it can overcome the limited transmission (Tx) capacity of mobile communication systems.
The number of transmission (Tx) antennas in a transmitter is NT, and the number of reception (Rx) antennas in a receiver is NR. In this way, theoretical channel transmission capacity of the MIMO communication system increases when both the transmitter and the receiver use a plurality of antennas, as compared to another case in which only the transmitter or the receiver uses several antennas. The theoretical channel transmission capacity of the MIMO communication system increases in proportion to the number of antennas. Therefore, transfer rate and frequency efficiency are greatly increased. Provided that a maximum transfer rate acquired when a single antenna is used is set to Ro, a transfer rate acquired when multiple antennas are used can theoretically increase by a predetermined amount that corresponds to the maximum transfer rate (Ro) multiplied by a rate of increase Ri.
For example, provided that a MIMO system uses four transmission (Tx) antennas and four reception (Rx) antennas, the MIMO system can theoretically acquire a high transfer rate which is four times higher than that of a single antenna system. After the above-mentioned theoretical capacity increase of the MIMO system was demonstrated in the mid-1990s, many developers began to conduct intensive research into a variety of technologies which can substantially increase a data transfer rate using such theoretical capacity increase. Some of the above technologies have been implemented in a variety of wireless communication standards, for example, a next-generation wireless LAN, etc.
The MIMO system uses a plurality of Tx antennas and a plurality of Rx antennas, and can overcome the fading influence generated in a radio frequency (RF) channel through a plurality of Tx/Rx paths. Therefore, the MIMO system can increase data transfer rate and transmission quality as compared to a single antenna system. However, the MIMO system requires a sufficiently long distance between a plurality of antennas so as to obtain a high transfer rate. The base station (BS) transmits and receives signals within a large coverage, so that antennas spaced apart from each other by a sufficiently long distance can be installed between the BSs. However, actually, a miniaturized user equipment (UE) has difficulty in guaranteeing a sufficiently long distance. Therefore, a Multi-User MIMO system in which UEs having a single antenna can communicate with the BS having multiple antennas is being intensively researched as part of LTE-Advanced of the 3GPP.
In the multi-cell environment, a transfer rate and quality of a UE located at a cell edge are considerably deteriorated by an inter-cell interference (ICI) generated from neighbor cells. In order to overcome this issue, a frequency reuse scheme for reducing interference by allocating orthogonal frequency resources between contiguous cells may be used. However, the frequency reuse scheme can improve a transfer rate and quality of a UE located at a cell edge, and at the same time can deteriorate the transfer rate and quality of a total network. In order to address this issue, a Coordinated Multi-Point (CoMP) scheme based on coordination between multiple cells may be used to reduce ICI by efficiently employing frequency resources. This scheme forms a virtual MIMO system by exchanging channel information or data between multiple cells.
Channel information between antennas is needed for efficient data transmission/reception in the MU-MIMO system. A time division cellular system allows the BS to allocate pilot resources to a UE so as to obtain such channel information. The UE transmits a predetermined pilot sequence to the BS through the allocated pilot resources, and the BS may estimate channel information upon receiving pilots. The amount of necessary pilot resources increases in proportion to the number of UEs, and the amount of pilot resources is limited, so that it is impossible to allocate orthogonal pilot resources among multiple cells to all UEs. Therefore, it is impossible to perfectly estimate a UE channel due to the reuse of pilots, and Tx/Rx filters based on the estimated channel may generate unexpected interference. Performance deterioration caused by reuse of pilot resources may become serious in a cooperative cellular environment.
Therefore, in order to obtain a high transfer rate and high quality by suppressing interference in a multi-cell cooperative communication system, a process for controlling interference generated from pilots is required for the multi-cell cooperative communication system. In other words, the BS must allocate optimum pilot resources to each UE, and the BS must estimate a channel between UEs using given pilot resources, so that the BS must design Tx/Rx filters on the basis of the estimated channel. In addition, in order to maximize a transfer rate, a method for designing an optimum scheme between the above-mentioned steps is of importance. Therefore, a method for designing the improved Tx/Rx filter capable of removing inter-cell interference (ICI) and/or interference between UEs on the basis of the estimated channel is needed.